Powder metallurgy is very important in the production of catalysts and or sintered bodies, such as metal filters, and in the fabrication of novel alloy systems and dispersion-hardened materials.
Powder metallurgy can also be used in the production of composite materials, which are used mainly in the electronics field and in which firm bonds are required between components that are immiscible in a liquid state. Such composites may comprise ceramic-metal, plastic-metal and metal-metal combinations.
The processes of producing metal powders comprises electrodeposition, the spraying of molten metals, and chemical precipitation, and result in powders which have different properties. Very fine powders are mainly obtained by chemical precipitation.
It is known that metal powders can be precipitated by a reduction of metal salt-containing solutions, e.g., with hydrogen (Sherrit-Gordon process). But that process results in particle size distributions in relative wide ranges and in particles having different shapes. Whereas the particle size distribution in the production of copper powder can be influenced by additives, such as polymeric amino compounds (Published German Application No. 26 53 281, U.S. Pat. No. 4,018,595) or ethylene/maleic anhydride copolymers (Published German Application No. 21 32 173, U.S. Pat. No. 3,694,185), the averge particle size of the resulting powder will always exceed 10 m.
From U.S. Pat. No. 4,539,041, it is known to reduce compounds of nonferrous metals, such as Au, Pd, Pt, Ir, Os, Cu, Ag, Ni, Co, Pb or Od in virtually anhydrous polyols to metals at temperatures of at least 85.degree. C. and up to 350.degree. C. The precipitate generally has a particle size between 0.1 and 10 .mu.m.
Disadvantages of the process are the fact that high temperatures are required to obtain a particle size below 0.5 .mu.m and that the reducing agents which can be used are restricted to polyols which are liquid at a reduction temperatures.
Another disadvantage of the process is the high consumption of expensive chemicals in an amount which is more than 20 times of the amount of copper that is produced.
From "Aust. Chem. Eng.", November 1983, pages 9 to 15, it is also known that copper sulfate in acid solutions can be reduced to fine copper powders by a treatment with starch or various sugars at pH values below 3.2 and in concentrations of 16 q/1 copper whereas basic sulfates will be formed and only a reduction to the Cu(I) oxide can be effected at pH values above 2.9.
That process has the disadvantage that the product is contaminated with sulfur owing to the sulfate content of the solution and the quantity of copper which can be produced per unit volume of the solution is limited by the solubility of copper sulfate.